1. Field
The present disclosure is directed to a method and apparatus for signaling aperiodic channel state indication reference signals. More particularly, the present disclosure is directed to a method and apparatus for signaling aperiodic channel state indication reference signals for Long Term Evolution (LTE) operation in unlicensed spectrum.
2. Introduction
Presently, users use portable devices, otherwise known as User Equipment (UE), such as smartphones, cell phones, tablet computers, selective call receivers, and other wireless communication devices, on LTE networks. Users use the UEs to download files, music, e-mail messages, and other data, as well as to watch streaming video, play streaming music, play games, surf the web, and engage in other data intensive activities. Because of large amounts of downloaded data as well as large amounts of users, LTE carriers can now use unlicensed spectrum to complement the bandwidth of their LTE networks to provide faster data to users. This allows the users to download data faster on their portable devices. For example, unlicensed spectrum can include spectrum at 5 GHz (e.g. used by WiFi) and other unlicensed spectrum. LTE technology can be deployed in unlicensed spectrum using the carrier aggregation framework wherein the primary cell uses licensed spectrum, and a secondary cell is deployed in the unlicensed spectrum. Transmissions on the unlicensed carrier typically have to follow Discontinuous Transmission requirements (DCT requirements) due to regulatory requirements and due the need to co-exist with other wireless systems operating in the same spectrum, such as Wi-Fi systems, LTE devices, such as User Equipment (UE), and base stations, such as Enhanced Node-B's (eNBs). In some regulations, a LTE device may be required to perform listen-before-talk (LBT) prior to transmitting on a carrier. If the device finds that the channel is busy, then it should defer its transmission until the carrier become clear. For DCT, after acquiring the channel, a LTE device can continuously transmit for X ms, where X=4 ms for some regulations and X=up to 13 ms for other regulations. After X ms the device has to cease transmission for some duration, sometimes referred as an idle period, perform LBT channel assessment, and reinitiate transmission only if LBT is successful. As a result of the discontinuous transmission, transmission of frames and reference signals in unlicensed spectrum can be aperiodic.
Unfortunately, because the transmission of frames and reference signals are aperiodic, a UE has problems determining how to measure a reference signal to ascertain channel state information to report it to a base station.
Thus, there is a need for a method and apparatus for signaling channel state information reference signals for LTE operation.